fix: Unpin SAML library. (#32167)

We were using an old version of the python3-saml library,
which was causing issues with newer versions of social-core.

The reason it was pinned was because our etree implementation
didn't support several fields that the saml library did, so
we are now importing those entities as well.

openedx/core/lib/safe_lxml/etree.py

@@ -16,7 +16,9 @@
 
 from lxml.etree import XMLParser as _XMLParser
 from lxml.etree import *  # lint-amnesty, pylint: disable=redefined-builtin
-from lxml.etree import _Element, _ElementTree
+# These private elements are used in some libraries to also defuse xml exploits for their own purposes.
+# We need to re-expose them so that the libraries still work.
+from lxml.etree import _Comment, _Element, _ElementTree, _Entity, _ProcessingInstruction
 
 # This should be imported after lxml.etree so that it overrides the following attributes.
 from defusedxml.lxml import XML, fromstring, parse

requirements/constraints.txt

@@ -40,9 +40,6 @@ matplotlib<3.4.0
 # Major upgrade will be done in separate ticket.
 pymongo<4.0.0
 
-# python3-saml==1.10.0 version started breaking a11y tests
-python3-saml<1.10.0
-
 # greater version has breaking changes and requires some migration steps.
 django-webpack-loader==0.7.0